It is important to increase the yield of chemical pulp for effective utilization of wood resources. A polysulfide cooking process is one of the techniques to increase the yield of kraft pulp as the most common type of chemical pulp.
The cooking liquor for the polysulfide cooking process is produced by oxidizing an alkaline aqueous solution containing sodium sulfide, i.e. so-called white liquor, by molecular oxygen such as air in the presence of a catalyst such as active carbon (e.g. reaction formula 1) (JP-A-61-259754 and JP-A-53-92981). By this method, a polysulfide cooking liquor having a polysulfide sulfur concentration of about 5 g/l can be obtained at a selectivity of about 60% and a conversion of 60% based on the sulfide ions. Here, polysulfide sulfur which may also be referred to as PS-S, is meant for sulfur of 0 valency in e.g. sodium polysulfide Na.sub.2 S.sub.x, i.e. sulfur of (x-1) atom. Further, the sulfide ion composing polysulfide ion which may also be referred to as Na.sub.2 S as S, is meant for sulfur corresponding to sulfur having oxidation number of -2 in the polysulfide ions, or sulfur of one atom per S.sub.x.sup.2-. Further, in this specification, the unit liter for the volume will be represented by l. However, by this air oxidation method, thiosulfate ions not useful for cooking, are likely to form by side reactions (e.g. reaction formulas 2 and 3), whereby it used to be difficult to produce a polysulfide cooking liquor at a high PS-S concentration with a high selectivity. EQU 4Na.sub.2 S+O.sub.2 +2H.sub.2 O.fwdarw.2Na.sub.2 S.sub.2 +4NaOH (1) EQU 2Na.sub.2 S+2O.sub.2 +H.sub.2 O.fwdarw.Na.sub.2 S.sub.2 O.sub.3 +2NaOH (2) EQU 2Na.sub.2 S.sub.2 +3O.sub.2 .fwdarw.2Na.sub.2 S.sub.2 O.sub.3 (3)
On the other hand, PCT International Publication WO95/00701 discloses a method for electrolytically producing a polysulfide cooking liquor. In this method, as an anode, an electrode substrate surface-coated with one or more oxides of ruthenium, iridium, platinum and palladium, is used. Specifically, a three-dimensional mesh electrode composed of a plurality of expanded-metal layers is disclosed as the anode.